Molecular chaperones of the Heat Shock Protein 70(Hsp7O) family bind unfolded polypeptide substrates to stabilize or alter their conformation and hydrolyze ATP to facilitate polypeptide release. The ATP hydrolysis of Hsp70 is regulated by another protein chaperon family Heat Shock Protein 40(Hsp4O)(1). It has been reported that Hsp4O may bind polypeptides with secondary structure, interact directly with Hsp70 and stimulate the ATP hydrolysis of Hsp70. Hsp4O itself can also bind denatured polypeptides and refold them as an independent protein chaperone(2). Sis I is a member of Hsp4O protein family in yeast Saccharomyces cerevisiae and it is essential for cell viability(3). We have crystallized the protein Sis I recently, the crystals diffract to 2.7A at SSRL station 7- 1. We propose to solve the crystal structure of Sis I by MAD method on BioCARS Station 14 BM-D.